Role of heat stress transcription factor HsfA5 as specific repressor of HsfA4

Unlike other eukaryotes, plants possess a complex family of heat stress transcription factors (Hsfs) with usually more than 20 members. Among them, Hsfs A4 and A5 form a group distinguished from other Hsfs by structural features of their oligomerization domains and by a number of conserved signature sequences. We show that A4 Hsfs are potent activators of heat stress gene expression, whereas A5 Hsfs act as specific repressors of HsfA4 activity. The oligomerization domain of HsfA5 alone is necessary and sufficient to exert this effect. Due to the high specificity of the oligomerization domains, other class A Hsfs are not affected. Pull-down assay and yeast two-hybrid interaction tests demonstrate that the tendency to form HsfA4/A5 heterooligomers is stronger than the formation of homooligomers. The specificity of interaction between Hsfs A4 and A5 was confirmed by bimolecular fluorescence complementation experiments. The major role of the representatives of the HsfA4/A5 group, which are not involved in the conventional heat stress response, may reside in cell type-specific functions connected with the control of cell death triggered by pathogen infection and/or reactive oxygen species.

Heat stress response in plants: a complex game with chaperones and more than twenty heat stress transcription factors

Compared to the overall multiplicity of more than 20 plant Hsfs, detailed analyses are mainly restricted to tomato and Arabidopsis and to three important representatives of the family (Hsfs A1, A2 and B1). The three Hsfs represent examples of striking functional diversification specialized for the three phases of the heat stress (hs) response (triggering, maintenance and recovery). This is best illustrated for the tomato Hsf system: (i) HsfA1a is the master regulator responsible for hs-induced gene expression including synthesis of HsfA2 and HsfB1. It is indispensible for the development of thermotolerance. (ii) Although functionally equivalent to HsfA1a, HsfA2 is exclusively found after hs induction and represents the dominant Hsf, the "working horse" of the hs response in plants subjected to repeated cycles of hs and recovery in a hot summer period. Tomato HsfA2 is tightly integrated into a network of interacting proteins (HsfA1a, Hsp17-CII, Hsp17-CI) influencing its activity and intracellular distribution. (iii) Because of structural peculiarities, HsfB1 acts as coregulator enhancing the activity of HsfA1a and/or HsfA2. But in addition, it cooperates with yet to be identified other transcription factors in maintaining and/or restoring housekeeping gene expression.

Tomato heat stress protein Hsp16.1-CIII represents a member of a new class of nucleocytoplasmic small heat stress proteins in plants

We describe a new class of plant small heat stress proteins (sHsps) with dominant nuclear localization (Hsp17-CIII). The corresponding proteins in tomato, Arabidopsis, and rice are encoded by unique genes containing a short intron in the beta4-encoding region of the alpha-crystallin domain (ACD). The strong nuclear localization results from a cluster of basic amino acid residues in the loop between beta5 and beta6 of the ACD. Using yeast 2-hybrid tests, analyses of native complexes of the sHsps, and immunofluorescence data, we demonstrate that, in contrast to earlier observations (Kirschner et al 2000), proteins of the sHsp classes CI, CII, and CIII interact with each other, thereby influencing oligomerization state and intracellular localization.

Role of Hsp17.4-CII as coregulator and cytoplasmic retention factor of tomato heat stress transcription factor HsfA2

HsfA2 is a heat stress (hs)-induced Hsf in peruvian tomato (Lycopersicon peruvianum) and the cultivated form Lycopersicon esculentum. Due to the high activator potential and the continued accumulation during repeated cycles of heat stress and recovery, HsfA2 becomes a dominant Hsf in thermotolerant cells. The formation of heterooligomeric complexes with HsfA1 leads to nuclear retention and enhanced transcriptional activity of HsfA2. This effect seems to represent one part of potential molecular mechanisms involved in its activity control. As shown in this paper, the activity of HsfA2 is also controlled by a network of nucleocytoplasmic small Hsps influencing its solubility, intracellular localization and activator function. By yeast two-hybrid interaction and transient coexpression studies in tobacco (Nicotiana plumbaginifolia) mesophyll protoplasts, we found that tomato (Lycopersicon esculentum) Hsp17.4-CII acts as corepressor of HsfA2. Given appropriate conditions, both proteins together formed large cytosolic aggregates which could be solubilized in presence of class CI sHsps. However, independent of the formation of aggregates or of the nucleocytoplasmic distribution of HsfA2, its transcriptional activity was specifically repressed by interaction of Hsp17.4-CII with the C-terminal activator domain. Although not identical in all aspects, the situation with the highly expressed, heat stress-inducible Arabidopsis HsfA2 was found to be principally similar. In corresponding reporter assays its activity was repressed in presence of AtHsp17.7-CII but not of AtHsp17.6-CII or LpHsp17.4-CII.

Tomato heat stress protein Hsp16.1-CIII represents a member of a new class of nucleocytoplasmic small heat stress proteins in plants

We describe a new class of plant small heat stress proteins (sHsps) with dominant nuclear localization (Hsp17-CIII). The corresponding proteins in tomato, Arabidopsis, and rice are encoded by unique genes containing a short intron in the beta4-encoding region of the alpha-crystallin domain (ACD). The strong nuclear localization results from a cluster of basic amino acid residues in the loop between beta5 and beta6 of the ACD. Using yeast 2-hybrid tests, analyses of native complexes of the sHsps, and immunofluorescence data, we demonstrate that, in contrast to earlier observations (Kirschner et al 2000), proteins of the sHsp classes CI, CII, and CIII interact with each other, thereby influencing oligomerization state and intracellular localization.

Selective activation of the developmentally regulated Ha hsp17.6 G1 promoter by heat stress transcription factors

Using two well-characterized heat stress transcription factors (Hsfs) from tomato (Lycopersicon peruvianum; LpHsfA1 and LpHsfA2), we analyzed the transcriptional activation of the Ha hsp17.6 G1 promoter in sunflower (Helianthus annuus) embryos. In this system, we observed transient promoter activation only with LpHsfA2. In contrast, both factors were able to activate mutant versions of the promoter with improved consensus Hsf-binding sites. Exclusive activation by LpHsfA2 was also observed in yeast (Saccharomyces cerevisiae) without other Hsfs and with a minimal Cyc1 promoter fused to the Ha hsp17.6 G1 heat stress cis-element. Furthermore, the same promoter mutations reproduced the loss of activation selectivity, as observed in sunflower embryos. The results of in vitro binding experiments rule out differential DNA binding of the two factors as the explanation for the observed differential activation capacity. We conclude that the specific sequence of this heat stress cis-element is crucial for Hsf promoter selectivity, and that this selectivity could involve preferential transcriptional activation following DNA binding. In sunflower embryos, we also observed synergistic transcriptional activation by co-expression of LpHsfA1 and LpHsfA2. Mutational analyses of the Ha hsp17.6 G1 promoter, combined with in vitro binding assays, suggest that mixed oligomers of the two factors may be involved in promoter activation. We discuss the relevance of our observations for mechanisms of developmental regulation of plant heat stress protein genes.

In the complex family of heat stress transcription factors, HsfA1 has a unique role as master regulator of thermotolerance in tomato

We generated transgenic tomato plants with altered expression of heat stress transcription factor HsfA1. Plants with 10-fold overexpression of HsfA1 (OE plants) were characterized by a single HsfA1 transgene cassette, whereas plants harboring a tandem inverted repeat of the cassette showed cosuppression (CS plants) by posttranscriptional silencing of the HsfA1 gene connected with formation of small interfering RNAs. Under normal growth conditions, major developmental parameters were similar for wild-type (WT), OE, and CS plants. However, CS plants and fruits were extremely sensitive to elevated temperatures, because heat stress-induced synthesis of chaperones and Hsfs was strongly reduced or lacking. Despite the complexity of the plant Hsf family with at least 17 members in tomato, HsfA1 has a unique function as master regulator for induced thermotolerance. Using transient reporter assays with mesophyll protoplasts from WT tomato, we demonstrated that plasmid-encoded HsfA1 and HsfA2 were well expressed. However, in CS protoplasts the cosuppression phenomenon was faithfully reproduced. Only transformation with HsfA2 expression plasmid led to normal expression of the transcription factor and reporter gene activation, whereas even high amounts of HsfA1 expression plasmids were silenced. Thermotolerance in CS protoplasts was restored by plasmid-borne HsfA2, resulting in expression of chaperones, thermoprotection of firefly luciferase, and assembly of heat stress granules.

The expanding family of Arabidopsis thaliana small heat stress proteins and a new family of proteins containing alpha-crystallin domains (Acd proteins)

Comprehensive analysis of the Arabidopsis genome revealed a total of 13 sHsps belonging to 6 classes defined on the basis of their intracellular localization and sequence relatedness plus 6 ORFs encoding proteins distantly related to the cytosolic class Cl or the plastidial class of sHsps. The complexity of the Arabidopsis sHsp family far exceeds that in any other organism investigated to date. Furthermore, we have identified a new family of ORFs encoding multidomain proteins that contain one or more regions with homology to the ACD (Acd proteins). The functions of the Acd proteins and the role of their ACDs remain to be investigated.

The expanding family of Arabidopsis thaliana small heat stress proteins and a new family of proteins containing alpha-crystallin domains (Acd proteins)

Comprehensive analysis of the Arabidopsis genome revealed a total of 13 sHsps belonging to 6 classes defined on the basis of their intracellular localization and sequence relatedness plus 6 ORFs encoding proteins distantly related to the cytosolic class Cl or the plastidial class of sHsps. The complexity of the Arabidopsis sHsp family far exceeds that in any other organism investigated to date. Furthermore, we have identified a new family of ORFs encoding multidomain proteins that contain one or more regions with homology to the ACD (Acd proteins). The functions of the Acd proteins and the role of their ACDs remain to be investigated.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

The expanding family of Arabidopsis thaliana small heat stress proteins and a new family of proteins containing alpha-crystallin domains (Acd proteins)

Comprehensive analysis of the Arabidopsis genome revealed a total of 13 sHsps belonging to 6 classes defined on the basis of their intracellular localization and sequence relatedness plus 6 ORFs encoding proteins distantly related to the cytosolic class Cl or the plastidial class of sHsps. The complexity of the Arabidopsis sHsp family far exceeds that in any other organism investigated to date. Furthermore, we have identified a new family of ORFs encoding multidomain proteins that contain one or more regions with homology to the ACD (Acd proteins). The functions of the Acd proteins and the role of their ACDs remain to be investigated.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

The expanding family of Arabidopsis thaliana small heat stress proteins and a new family of proteins containing alpha-crystallin domains (Acd proteins)

Comprehensive analysis of the Arabidopsis genome revealed a total of 13 sHsps belonging to 6 classes defined on the basis of their intracellular localization and sequence relatedness plus 6 ORFs encoding proteins distantly related to the cytosolic class Cl or the plastidial class of sHsps. The complexity of the Arabidopsis sHsp family far exceeds that in any other organism investigated to date. Furthermore, we have identified a new family of ORFs encoding multidomain proteins that contain one or more regions with homology to the ACD (Acd proteins). The functions of the Acd proteins and the role of their ACDs remain to be investigated.

Plants contain a novel multi-member class of heat shock factors without transcriptional activator potential

Based on phylogeny of DNA-binding domains and the organization of hydrophobic repeats, two families of heat shock transcription factors (HSFs) exist in plants. Class A HSFs are involved in the activation of the heat shock response, but the role of class B HSFs is not clear. When transcriptional activities of full-length HSFs were monitored in tobacco protoplasts, no class B HSFs from soybean or Arabidopsis showed activity under control or heat stress conditions. Additional assays confirmed the finding that the class B HSFs lacked the capacity to activate transcription. Fusion of a heterologous activation domain from human HSF1 (AD2) to the C-terminus of GmHSFB1-34 gave no evidence of synergistic enhancement of AD2 activity, which would be expected if weak activation domains were present. Furthermore, activity of AtHSFB1-4 (class B) was not rescued by coexpression with AtHSFA4-21 (class A) indicating that the class A HSF was not able to provide a missing function required for class B activity. The transcriptional activation potential of Arabidopsis AtHSFA4-21 was mapped primarily to a 39 amino acid fragment in the C-terminus enriched in bulky hydrophobic and acidic residues. Deletion mutagenesis of the C-terminal activator regions of tomato and Arabidopsis HSFs indicated that these plant HSFs lack heat-inducible regulatory regions analogous to those of mammalian HSF1. These findings suggest that heat shock regulation in plants may differ from metazoans by partitioning negative and positive functional domains onto separate HSF proteins. Class A HSFs are primarily responsible for stress-inducible activation of heat shock genes whereas some of the inert class B HSFs may be specialized for repression, or down-regulation, of the heat shock response.

Isolation and characterization of HsfA3, a new heat stress transcription factor of Lycopersicon peruvianum

Stress-induced transcription of heat shock proteins (Hsps) in eukaryotes is mediated by a conserved class of transcription factors called heat stress transcription factors (Hsfs). Here we report the isolation and functional characterization of HsfA3, a new member of the Hsf family. HsfA3 was cloned from a tomato heat stress cDNA library by yeast two-hybrid screening, using HsfA1 as a bait. HsfA3 is a single-copy gene with all the conserved sequence elements characteristic of a heat stress transcription factor. The constitutively expressed HsfA3 is mainly found in the cytoplasm under control conditions and in the nucleus under heat stress conditions. Functionally, HsfA3 behaves similarly to the already known members of tomato Hsf family. It is able to substitute yeast Hsf for viability functions and is a strong activator of Hsf-dependent reporter constructs both in tobacco protoplasts and yeast. Finally, similar to the AHA motifs in HsfA1 and HsfA2, the activator function depends on four short peptide motifs with a central tryptophan residue found in the C-terminal domain of HsfA3.

The tomato Hsf system: HsfA2 needs interaction with HsfA1 for efficient nuclear import and may be localized in cytoplasmic heat stress granules

In heat-stressed (HS) tomato (Lycopersicon peruvianum) cell cultures, the constitutively expressed HS transcription factor HsfA1 is complemented by two HS-inducible forms, HsfA2 and HsfB1. Because of its stability, HsfA2 accumulates to fairly high levels in the course of a prolonged HS and recovery regimen. Using immunofluorescence and cell fractionation experiments, we identified three states of HsfA2: (i) a soluble, cytoplasmic form in preinduced cultures maintained at 25 degrees C, (ii) a salt-resistant, nuclear form found in HS cells, and (iii) a stored form of HsfA2 in cytoplasmic HS granules. The efficient nuclear transport of HsfA2 evidently requires interaction with HsfA1. When expressed in tobacco protoplasts by use of a transient-expression system, HsfA2 is mainly retained in the cytoplasm unless it is coexpressed with HsfA1. The essential parts for the interaction and nuclear cotransport of the two Hsfs are the homologous oligomerization domain (HR-A/B region of the A-type Hsfs) and functional nuclear localization signal motifs of both partners. Direct physical interaction of the two Hsfs with formation of relatively stabile hetero-oligomers was shown by a two-hybrid test in Saccharomyces cerevisiae as well as by coimmunoprecipitation using tomato and tobacco whole-cell lysates.

Heat stress transcription factors from tomato can functionally replace HSF1 in the yeast Saccharomyces cerevisiae

The fact that yeast HSF1 is essential for survival under nonstress conditions can be used to test heterologous Hsfs for the ability to substitute for the endogenous protein. Our results demonstrate that like Hsf of Drosophila, tomato Hsfs A1 and A2 can functionally replace the corresponding yeast protein, but Hsf B1 cannot. In addition to survival at 28 degrees C, we checked the transformed yeast strains for temperature sensitivity of growth, induced thermotolerance and activator function using two different lacZ reporter constructs. Tests with full-length Hsfs were supplemented by assays using mutant Hsfs lacking parts of their C-terminal activator region or oligomerization domain, or containing amino acid substitutions in the DNA-binding domain. Remarkably, results with the yeast system are basically similar to those obtained by the analysis of the same Hsfs as transcriptional activators in a tobacco protoplast assay. Most surprising is the failure of HsfB1 to substitute for the yeast Hsf. The defect can be overcome by addition to HsfB1 of a short C-terminal peptide motif from HsfA2 (34 amino acid residues), which represents a type of minimal activator necessary for interaction with the yeast transcription apparatus. Deletion of the oligomerization domain (HR-A/B) does not interfere with Hsf function for survival or growth at higher temperatures. But monomeric Hsf has a markedly reduced affinity for DNA, as shown by lacZ reporter and band-shift assays.

Intracellular distribution and identification of the nuclear localization signals of two plant heat-stress transcription factors

Similar to heat-stress transcription factors (HSFs) from non-plant sources, HSFA1 and HSFA2 from tomato (Lycopersicon esculentum Mill) contain two conserved clusters of basic amino acid residues (K/R1 and K/R2) which might serve as nuclear localization signal (NLS) motifs. Mutation of either one of them and functional testing of the corresponding proteins in a transient expression assay using tobacco (Nicotiana plumbaginifolia L:) protoplasts gave the following results. Whereas K/R1, positioned in all HSFs at the C-terminus of the DNA-binding domain, had no influence on nuclear import, the K/R1 mutants were impaired in their interaction with the DNA (band-shift assays). In contrast to this, mutants of the K/R2 motif, found 15-20 amino acid residues C-terminal of the oligomerization domain (HR-A/B region), had wild-type activity in DNA-binding but were defective in nuclear import. Thus, for the related tomato HSFA1 and HSFA2 the K/R2 cluster represents the only NLS motif, and in this function it cannot be replaced by K/R1.

The Hsf world: classification and properties of plant heat stress transcription factors

Based on the partial or complete sequences of 14 plant heat stress transcription factors (Hsfs) from tomato, soybean, Arabidopsis and maize we propose a general nomenclature with two basic classes, i.e. classes A and B each containing two or more types of Hsfs (HsfA1, HsfA2 etc.). Despite some plant-specific peculiarities, essential functional domains and modules of these proteins are conserved among plants, yeast, Drosophila and vertebrates. A revised terminology of these parts follows recommendations agreed upon among the authors and representatives from other laboratories working in this field (see legend to Fig. 1). Similar to the situation with the small heat shock proteins (sHsps), the complexity of the hsf gene family in plants appears to be higher than in other eukaryotic organisms.

The Hsf world: classification and properties of plant heat stress transcription factors

Based on the partial or complete sequences of 14 plant heat stress transcription factors (Hsfs) from tomato, soybean, Arabidopsis and maize we propose a general nomenclature with two basic classes, i.e. classes A and B each containing two or more types of Hsfs (HsfA1, HsfA2 etc.). Despite some plant-specific peculiarities, essential functional domains and modules of these proteins are conserved among plants, yeast, Drosophila and vertebrates. A revised terminology of these parts follows recommendations agreed upon among the authors and representatives from other laboratories working in this field (see legend to Fig. 1). Similar to the situation with the small heat shock proteins (sHsps), the complexity of the hsf gene family in plants appears to be higher than in other eukaryotic organisms.

Solution structure of the DNA-binding domain of the tomato heat-stress transcription factor HSF24

Two-dimensional-NMR and three-dimensional-NMR experiments were performed to determine the solution structure of the DNA-binding domain of the tomato heat-stress transcription factor HSF24. Samples of uniformly 15N-labeled and 15N, 13C-labeled recombinant proteins were used in the investigation. A near-complete assignment of the backbone 1H, 15N, and 13C resonances was obtained by three-dimensional triple-resonance experiments, whereas three-dimensional 15N-TOCSY-heteronuclear-single-quantum-correlation-spectroscopy, HCCH-COSY and HCCH-TOCSY spectra were recorded for side-chain assignments, 885 non-redundant distance constraints from two-dimensional-homonuclear and three-dimensional-15N-edited and 13C-edited NOESY spectra and 40 hydrogen-bond constraints from exchange experiments were used for structure calculations. The resulting three-dimensional structure contains a three-helix bundle and a small four-stranded antiparallel beta-sheet that forms a hydrophobic core. The two C-terminal helices are parts of a highly conserved helix-turn-helix motif that is probably involved in DNA recognition and binding. In contrast to heat-stress factors from yeast and animals, the plant heat-stress factors lack a loop of 11 amino acid residues inserted between beta3 and beta4. This leads to a tight turn between these beta-strands.